[CMake][llvm] avoid changing global flags (may be used outside of llvm)

[radford]

Summary

Changing global flags can break builds of projects that include/build
llvm as a sub-project, as the effect is global. Ideally we would
disable this warning at the directory level instead, but the obvious
way (disabling warning:D9025) isn't supported. At least we can limit
the effect to only MSVC.

[rnk]

lgtm

llvm/cmake/modules/HandleLLVMOptions.cmake

if( NOT MSVC )
     add_definitions( -D_DEBUG )
   endif()
   # On non-Debug builds cmake automatically defines NDEBUG, so we
   # explicitly undefine it:
   if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
     # NOTE: use `add_compile_options` rather than `add_definitions` since
     # `add_definitions` does not support generator expressions.
     add_compile_options($<$<OR:$<COMPILE_LANGUAGE:C>,$<COMPILE_LANGUAGE:CXX>>:-UNDEBUG>)
-
+    if (MSVC)
       # Also remove /D NDEBUG to avoid MSVC warnings about conflicting defines.
       foreach (flags_var_to_scrub
           CMAKE_CXX_FLAGS_RELEASE
           CMAKE_CXX_FLAGS_RELWITHDEBINFO
           CMAKE_CXX_FLAGS_MINSIZEREL
           CMAKE_C_FLAGS_RELEASE
           CMAKE_C_FLAGS_RELWITHDEBINFO
           CMAKE_C_FLAGS_MINSIZEREL)
         string (REGEX REPLACE "(^| )[/-]D *NDEBUG($| )" " "
           "${flags_var_to_scrub}" "${${flags_var_to_scrub}}")
       endforeach()
      endif()
   endif()
+endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)
   add_definitions(-DEXPENSIVE_CHECKS)
 
   # In some libstdc++ versions, std::min_element is not constexpr when
   # _GLIBCXX_DEBUG is enabled.
   CHECK_CXX_SOURCE_COMPILES("
     #define _GLIBCXX_DEBUG